The knee joint basically consists of the bone interface of the distal end of the femur and the proximal end of the tibia. Appearing to cover or at least partially protect this interface is the patella, which is a sesamoid bone within the tendon of the long muscle (quadriceps) on the front of the thigh. This tendon inserts into the tibial tuberosity and the posterior surface of the patella is smooth and glides over the femur.
The femur is configured with two knob like processes (the medial condyle and the lateral condyle) which are substantially smooth and which articulate with the medial plateau and the lateral plateau of the tibia, respectively. The plateaus of the tibia are substantially smooth and slightly cupped thereby providing a slight receptacle for receipt of the femoral condyles.
When the knee joint is damaged whether as a result of an accident or illness, a prosthetic replacement of the damaged joint may be necessary to relieve pain and to restore normal use to the joint. Typically the entire knee joint is replaced by means of a surgical procedure that involves removal of the surfaces of the corresponding damaged bones and replacement of these surfaces with prosthetic implants. This replacement of a native joint with a prosthetic joint is referred to as a primary total-knee arthroplasty.
On occasion, the primary knee prostheses fails. Failure can result from many causes, including wear, aseptic loosening, osteolysis, ligamentous instability, arthrofibrosis and patellofemoral complications. When the failure is debilitating, revision knee surgery may be necessary. In a revision, the primary knee prosthesis is removed and replaced with components of a revision prosthetic knee system.
Knee implant systems for both primary and revision applications are available from a variety of manufacturers, including DePuy Orthopaedics, Inc. of Warsaw, Ind. DePuy and others offer several different systems for both primary and revision applications. For example, DePuy Orthopaedics offers the P.F.C. SIGMA® Knee System, the LCS® Total Knee System, and the S-ROM Modular Total Knee System. Each of these orthopaedic knee systems includes several components, some appropriate for use in primary knee arthroplasty and some appropriate for use in revision surgery.
DePuy Orthopaedics also offers other orthopaedic implant systems for other applications. One such system is the LPS System. The LPS System is provided for use in cases of severe trauma and disease. In such cases, the trauma or disease can lead to significant amounts of bone loss. The LPS System provides components that can replace all or significant portions of a particular bone, such as the femur. The DePuy LPS System is described more fully in U.S. patent application Ser. No. 10/135,791, entitled “Modular Limb Preservation System”, filed Apr. 30, 2002 by Hazebrouck et al., which is incorporated by reference herein in its entirety.
In some patients, bone deficiencies in the distal and posterior femur may make it difficult for the surgeon to re-establish the natural joint line and to provide equal flexion and extension gaps. Prosthetic knee implant systems have commonly included femoral augments for use on the distal and posterior bone-facing surfaces of the femoral implant components. Examples of such augments are disclosed in U.S. Pat. Nos. 6,005,018 and 5,984,969, which are incorporated by reference herein in their entireties. Such components serve to augment the inferior and posterior portions of the femoral component to add additional thickness to compensate for the lack of sufficient boney tissue, allowing the joint line to be distalized.
As disclosed in U.S. Pat. Nos. 6,005,018 and 5,984,969, such augments are commonly connected to the bone-facing surfaces of the distal femoral articulation components through screws or pins inserted superiorly or anteriorly from the bone-facing surfaces of the augments and threaded into threaded openings or collets or other receptacles received in openings in the bone-facing surface of the distal femoral articulation components. For the posterior augments, a special tool may be required to connect the augment to the posterior condylar bone-facing surface of the distal femoral articulation component; for example, a wobble driver tip to facilitate access across the femoral anterior flange.
After the augmented distal femoral articulation component is implanted, the augments and distal femoral articulation components cannot be disassembled until after the entire assembly is removed from the distal femur because the connection mechanisms are no longer accessible. During revision surgery, it may be advantageous to the surgeon to have the option of severing the connections between the distal femoral articulation component and the augments prior to removal of these components from the bone.